Yersina pestis, the ethiologic agent of plague or Black Death, has in historical times caused devastating pandemics unrivaled by any other infectious disease. Due to its very rapid replication and effective immune evading capacity, Yersinia pestis has recently been recognized as a potential tool for bioterrorism. In particular, an aerosolized delivery of the bacterium could cause a rapid and fulminant pneumonic infection, which subsequently may spread from person to person. Although several vaccines exist and Yersinia usually is sensitive to streptomycin and tetracycline type antibiotics, the pneumonic form of the disease is difficult to treat and still often results in death. Additional supportive treatments may significantly reduce this lethality, especially in the case of massive exposure of a population to weaponized Yersinia. This project focuses on a key component of the molecular machinery by which Yersinia pestis evades the immune system, a highly active protein tyrosine phosphatase (PTPase) termed YopH. In infected hosts, the bacteria multiply in lymph nodes, where they adhere to T and B cells and inject them with YopH. Inside the lymphocytes, YopH efficiently inhibits lymphocyte activation and the development of an immune response. We have found that YopH interferes with early T cell antigen receptor signaling and T cell survival. Our Specific Aims are: (1) Molecular targets of YopH in T cell antigen receptor signal transduction. These studies will attempt to determine the molecular mechanism by which YopH inhibits TCR-induced T cell activation using substrate-trapping technology, proteomics, tryptic peptide mapping, confocal microscopy, and functional assays and read-outs for T cell activation. (2) Mechanisms of cell cycle perturbation and cell death induced by YopH. We will investigate the role of nuclear YopH and the mechanism of YopH-induced cell death by a combination of imaging, biochemistry, proteomics, and functional assays for cell death. (3) Development of a YopH-specific inhibitor. In close collaborations within this P01, we will develop YopH-specific small molecule inhibitors by combinations of high through-put screening, NMR, virtual docking, and constrained peptide chemistry. Leads will be optimized and taken through counter-screening with other PTPases and functional tests in YopH-expressing T cells.